diff options
Diffstat (limited to 'sound/select.s')
| -rw-r--r-- | sound/select.s | 39 |
1 files changed, 39 insertions, 0 deletions
diff --git a/sound/select.s b/sound/select.s index 887a585..8f15399 100644 --- a/sound/select.s +++ b/sound/select.s @@ -152,6 +152,34 @@ lda #$00 sta APU::m_triangle_high + ;; The noise channel produces noise with a pseudo-random bit generator. Its + ;; envelope register is pretty much the same as with the square channels, + ;; but the two most significant bits are unused as "duty" is, like with the + ;; triangle channel, not a thing on this context. + lda #%00110000 + sta APU::m_noise_envelope + + ;; NOTE: address $400D is unused from the APU. + + ;; You then configure the mode of noise by writing to the $400E address. In the + ;; low nibble you configure the "noise period", which in turn is the index + ;; for a table that the APU will use as the real noise period (see: + ;; https://www.nesdev.org/wiki/APU_Noise for specifics). Finally, only the + ;; most significant bit is used on the high nibble, which sets the + ;; "mode". Long story short, it dramatically shortens the period if set, + ;; which will give you a more "metallic" sound. Change the current value + ;; from "$0A" to "$8A" and test it by yourself ;-) + lda #$0A + sta APU::m_noise_mode + + ;; Finally, there is the counter register for the noise channel. Given that + ;; we have disabled when we configured 'APU::m_noise_envelope', you'd think + ;; that you wouldn't have to touch it, but you'd be wrong. This register + ;; needs to be written at least once so to trigger the enablement of this + ;; channel. We do it now with a zero value, which also does the trick. + lda #0 + sta APU::m_noise_counter + cli lda #%10001000 sta PPU::m_control @@ -194,6 +222,17 @@ @set_triangle: sta APU::m_triangle_control + ;; Mute/Play the noise channel + lda Joypad::zp_buttons1 + and #%00010000 + beq @reset_noise + lda #%00111000 + bne @set_noise +@reset_noise: + lda #%00110000 +@set_noise: + sta APU::m_noise_envelope + ;; NOTE: done :) lda #%10000000 |